1. Field of the Invention
This invention relates to a grain-oriented electromagnetic steel sheet used as a core material of transformers and power generators, especially to a grain-oriented electromagnetic steel sheet having low iron loss and excellent strain resistance and excellent performance in use.
2. Description of the Related Art
Grain-oriented electromagnetic steel sheets containing Si having crystal grains oriented along the (110) {001} or (100) {001} direction are widely used for various kinds of iron cores operated at commercial frequencies because of good soft-magnetic properties. An important property required of this kind of electromagnetic steel sheet is low iron loss (generally represented by electric loss W.sub.17/50 (W/kg) when the steel sheet is magnetized to 1.7T at a frequency of 50 Hz).
Methods for reducing the iron loss of a steel sheet include increasing electric resistance by adding Si which is effective for reducing eddy current loss of a steel sheet, or reducing the thickness of the steel sheet, or making the grain diameter small, or aligning the orientation of grains that are effective for reducing hysteresis loss.
Among those methods, addition of Si encounters limitations since decrease of saturation magnetic flux density may be induced when the amount of Si is excessive, and expansion of iron core size is caused. Reducing the thickness of the steel sheet, on the other hand, tends to result in excessive production cost increase.
Accordingly, recent technical developments for reducing iron loss have concentrated on improving alignment of crystal orientations and reducing the grain size in the steel. The alignment of orientations can usually be evaluated by magnetic flux density B.sub.8 (T) at a magnetization strength of 800 A/m. However, the alignment of orientations should be optimized, i.e., the B.sub.8 value should be adjusted to its optimum in order to obtain minimum iron loss, because an inconsistent relationship exists wherein improving the alignment of crystal orientations inevitably results in an increase of grain diameter and hence deterioration of iron loss.
The requirement to make the grain diameter small for reducing the iron loss has been eliminated thanks to the recent technical development by which the width of magnetic domains can be finely divided artificially by irradiating with a plasma jet or laser beam. Therefore, the method for reducing the iron loss by increasing the alignment of orientations has became a leading technique today, allowing development of a material having a magnetic flux density (Be) of as large as 1.93 to 2.00T.
Processing methods developed for finely dividing magnetic domains include not only forming linear grooves or introducing linear local stress, but also smoothing the roughness of the interface between the surface of the steel sheet and the non-metallic coating film, or applying crystal orientation emphasis on the surface of the metal. Finely dividing the magnetic domains enabled some improvement of iron loss characteristics.
It is necessary that secondary recrystallization is perfectly controlled to enhance the alignment of orientations. In secondary recrystallization growth of normal crystal grains can be suppressed by finely dispersing precipitates of inhibitors such as AlN, MnSe or MnS, thereby allowing growth of large grains along a specified preferable ((110)[001]) direction and nearby directions referred to as Goss directions. Inhibitor elements tending to segregate at grain boundaries, such as Sb, Sn and Bi, are also used as sub-inhibitors.
Production of electromagnetic steel sheets having a high magnetic flux density as described above has involved combining the foregoing techniques with a technique adapted to control the aggregated textures of crystal grains.
When a transformer was produced using a grain-oriented electromagnetic steel sheet having good soft-magnetic properties, however, the transformer often failed to have the characteristics required for practical use. This is especially true in the case of a laminated transformer where the steel sheet is used without applying stress-relief annealing after shear processing, which causes discrepancies between the characteristics of the materials and especially the performance a large transformer. Performance in final usage is referred to herein generically as "performance of a practical device."
There have been problems in the prior art that expected characteristics suitable for practical devices cannot always be obtained even when a transformer is produced by using a grain-oriented electromagnetic steel sheet having a high magnetic flux density. This is an intrinsic problem when a material having a high magnetic flux density is used. It was elucidated that an undesirable distorted flow of the magnetic flux that causes digression of the magnetic flux from its flow direction takes place at the T-shaped junction of the transformer, so that reduction of the iron loss cannot be attained. This problem was considered to be beyond improvement.
However, the practical performance of a transformer or other device is largely deteriorated even when recent materials are used in which the flux density has been much more improved.
The phenomenon, wherein iron loss characteristics deteriorate under shear processing and lamination, was observed as being accompanied by improvement of magnetic flux density. This phenomenon is still under investigation. The only countermeasures now available at hand are to suppress addition of strain as much as possible, by careful handling of the material.
Although it is doubtless true that iron loss characteristics have been improved by various techniques for finely dividing magnetic domains as described above, yet there remain problems, since the desired characteristics cannot be attained when a practical device is produced using the materials now available, especially when the device is used in a high magnetic field.
The method step of imparting high magnetic flux density to the grain-oriented steel sheet has been known in the art and elements such as Al, Sb, Sn and Bi are effective for the purpose.
A value of 1.981T is reported in Japanese Examined Patent Publication No. 46-23820 as B.sub.10 (the magnetic flux density under a magnetic field strength of 1000 A/m) in a grain-oriented electromagnetic steel sheet containing Al and S, while a value of 1.95T is reported in Japanese Examined Patent Publication No. 62-56923 as B.sub.8 in a grain-oriented electromagnetic steel sheet containing Al, Se, Sb and Bi as inhibitors.
The magnetic properties of these grain-oriented electromagnetic steel sheets are splendid, but when a transformer is produced using these electromagnetic steel sheets having a desired value for iron loss of the resulting device cannot be often obtained. This is believed to originate, as hitherto described, from a high alignment of crystals that cannot be avoided.